Background art is found in U.S. Pat. Nos. 5,116,441; 5,425,827; 5,429,692; 5,512,116; 5,589,018; 5,614,046; 5,653,497; 5,860,689; 6,033,507 and 6,187,124. all of whose content is incorporated herein by reference.
Various adhesive products are commercially available in the adhesive industry. Many of which cannot and are not used for windshield repair because of issues in their curing process, weatherability, strength, color, clarity and viscosity. In the trade for the repair of automobile windshield breaks and cracks all of the known adhesive/resins in use today fall under the category of UV curing free radical acrylics. In this description and claims the terms “UV curing free radical acrylic resin” and “UV radical” and “UV radical acrylic” and “UV radical resin” and similar terms known in the art to have essentially the same meaning may be used interchangeably. The practicality of an acceptable windshield repair is based on several important factors which include that the index of refraction of the resin is substantially the same as that of the glass, that the bond is able to restore the structural integrity of the glass and will seal the laminate from the atmosphere and will prevent the damage from spreading.
A crack or break in glass is visible, referred to as refraction, because as light rays pass the boundary between the glass and the air in the crack, they are refracted or bent. Light that is reflected back from such boundaries comes at a different angle than if there were no crack and hence the eyes and brain of the observer sense the discontinuity. If two materials have the same index of refraction there will be no bending of the light rays at a boundary between the materials and an observer will not sense the discontinuity. Thus a crack filled with a resin that has the same index of refraction as the glass will essentially seem to disappear except for what is referred to as the “Head-On Angle” which is a hairline. The profile of the crack is the side view of the crack and this view is the angle in which the crack will virtually disappear. The head-on angle is just that and this angle is always visible. For example the head-on angle of a horizontal crack can be seen by lying down on the seat. Fortunately most long cracks run horizontally and that is why they are not visible from the normal sitting position but a star break or combination break will have different head-on angles for each crack within the break and this will be of concern when it is in the acute area in front of the driver.
There are different types of clear adhesives. Some are aerobic or “air drying”, anaerobic, two-part and others are cured or hardened by radiating them with ultraviolet light (UV) which is usually provided by a lamp made for this purpose. The windshield repair industry currently uses free radical UV curing resins (“UV radical resins”) that are also anaerobic (oxygen inhibited) with acrylic or acrylate components exclusively.
Aerobic resins come in two parts and are mixed before their use. Their curing begins from the time of mixing whereas the cure of the ultraviolet types begins when they are subjected to the ultraviolet light. Two-part resins were tried in earlier years but were discontinued because of the lack of control of the cure. The advantages of UV curing is that the curing process is able to be controlled so that it can begin when the technician has totally filled the break or crack with the resin. The time to completely fill the break or crack can vary depending upon the break or crack, from a couple of minutes to over thirty minutes. The aerobic resins may commence to cure before the technician is finished and ruin the repair. Therefore the windshield repair industry, which began in the mid 1970s, has adopted UV curing as the most efficient method and the type of UV curing has been free radical and the resin is and has been acrylic. Another type of UV curing adhesive, that has not been used by the windshield repair industry, is cationic epoxy mainly because the viscosity is too high to fit into a stone break. The viscosities for cracks are discussed in U.S. Pat. No. 5,116,441 and CIPs to Campfield. Generally another problem is that the polymerization is retarded by moisture.
Automobile windshields are made of a “sandwich” of two layers of glass laminated to an inner layer of material called polyvinylbutyral (PVB). The glass area for each layer has an exposed surface and an inner surface in contact with the inner plastic (PVB) layer. The outside glass layer is called the “outerlite” and the inside glass layer is called the “innerlite” and the laminate is called the interlayer.
There are two broad categories of breaks or cracks in windshields that occur to the outerlite. One category is called a “stone-break” and the other is called a crack, usually a long crack i.e. over six inches. These are shown schematically in FIG. 1, in which 1a is a stone break, 1b is a long crack of the floater type and 1c is an edge crack. These are described in more detail below.
Stone-Breaks
Stone breaks are seen as small breaks with a chip on the surface of the glass and are called a bullseye, a star-break, a combination bullseye and star break, and sometimes as variants on these configurations. It is well known that stone breaks are “un-surfaced” that is they extend below the surface of the glass and are actually on the inside of the outerlite against the PVB. Repair methods and equipment employ application of vacuum at an entry location which is the impact spot which caused the damage, followed by injection of resin under pressure to flow into the break. This is called “stone damage art”. In this manner, the resin replaces the air in the break.
Various tools have been proposed to aid in filling stone breaks with resin. One is described in U.S. Pat. No. 3,993,520 to Werner and utilizes an injector assembly having a piston inside a cylinder. Another is described in U.S. Pat. No. 5,512,116 to Campfield. When the piston is depressed the resin is injected out the open end of the cylinder and into the stone break. The assembly is held to the surface of the glass by suction cups. A similar apparatus with an adjustable arm is disclosed in U.S. Pat. No. 4,291,866 to Petersen. A variation of the Werner apparatus is in U.S. Pat. No. 4,569,808 to Smali while U.S. Pat. No. 4,744,841 to Thomas utilizes vibration and heat. An apparatus with a spring loaded air exit is disclosed in U.S. Pat. No. 4,775,305 to Alexander and U.S. Pat. No. 4,814,185 to Jones has a side tube for introduction of the resin. The use of vacuum to aid the resin injection is explored in U.S. Pat. No. 4,820,148 to Anderson and U.S. Pat. No. 4,919,602 to Janszen. Other related patents are U.S. Pat. No. 4,419,305 to Matles and U.S. Pat. No. 4,385,879 to Wilkinson
For a normal bullseye type stone break approximately 1 cc of low viscosity resin is needed. This is equivalent to 3-4 drops of resin, an amount sufficient to repair most stone breaks. Most stone breaks are repaired with resin in the viscosity range of 15-40 cps with 20 cps being the most popular.
Stone-breaks must be cured with the injector in the pressure mode with UV curing free radical acrylate resins in order to compensate for the shrinkage which can be as high as 20% to 30%. If it is not cured under pressure there will be a void in the middle due to the shrinkage and it will refract. The acrylic resin has a poor bond to the PVB. It has a bond to the glass and the glass will expand and contract from temperature change and the PVB will change from stiff to soft from temperature change. Consequently the resin can separate from the PVB eventually and cause a miniscule gap between the resin and the PVB that will refract, which is a cosmetic blemish. This will happen with un-garaged vehicles more so than with garaged vehicles.
After the stone break has been cured under pressure then the pit or chip on the surface must be filled with a drop of resin referred to as “pit filler” and the process referred to as ‘filling the pit’. Mylar film is placed on top of the pit and it is cured, i.e. a second curing. Often the resin has cured around the seal of the injector while curing under pressure and this must first be removed before the pit resin and Mylar tab are placed on the surface. After the pit resin is cured there is excess resin around the pit and it is scraped off with a razor blade and the pit which was clear is now opaque or white, which now requires another messy process of “polishing the pit”.
Cracks
There are two types of cracks, a floater crack and an edge crack and these cracks are referred to as a “short crack”, 6-inches or less and a “long crack”, which is over six-inches in length. These are clearly distinguished in the field as noted in the patents listed above. Prior to U.S. Pat. No. 5,116,441 to Campfield and continuations in part, the windshield repair industry did not repair long cracks because it was understood that the resin could not withstand the stress. This was true because repairs were made using one low viscosity acrylic and these resin's strength and durability comes from the thicker ingredient oligomer. Low viscosity resins are low in oligomer and higher in monomer. U.S. Pat. No. 5,116,441 and CIPs to Campfield solved this problem by injecting the thickest acrylic resins possible into the crack, which ended up being two or more viscosities (usually three) being injected into a crack from the surface. This method is referred to as the “multi-viscosity method” is licensed to many businesses and is known today as the best and only reliable method in the industry. This method uses UV curing free radical acrylic resins.
The floater crack is a stone-break that elongated into a floater crack from temperature change, such as a defroster on a cold day or a car wash on a hot day. Over 90% of stone-breaks will never crack and that is why only 10% of repaired cracks are floater cracks.
The floater crack is called a floater crack because it has not run to the edge and is floating in the middle section of the windshield. The floater crack is tight because the lamination stress is pulling it together. This crack will have two points and is repaired by drilling in front of each point, tapping a small bullseye and placing a repair tool at each one with a resin of low viscosity (20-45 cps). Crack openers will be placed on the inside to pry open the crack. Then another tool will be slid along the surface of the crack injecting a medium viscosity resin, of 200 cps. After the crack is completely filled with resin and has no spots, a bead of resin is placed on the surface of the crack and then Mylar tabs or strips are placed on top of the resin to remove access to oxygen and prevent oxygen inhibition of the curing process. The tools at the points are kept in place and then the first curing cycle is done. The tools at the points are left in the pressure mode to compensate for the shrinkage of the acrylic resin and the Mylar is to address the oxygen inhibition of the resin. After the resin has been cured the tools at the points are removed, Mylar is placed over the drilled holes and another cured is done. After the second curing the Mylar strips are removed, resin is scraped off the windshield and the mess is cleaned up. The Mylar tabs or strips will be a mess and will be wiped off or placed in a jar of alcohol so they can be cleaned and used again or they are thrown out.
The edge crack is 90% of repairable cracks and 95% of edge cracks will be long cracks (over six inches). The edge crack is also the cause of approximately 80% of windshield replacements, see U.S. Pat. No. 5,653,497 to Campfield. The reason for this is a manufacturing quality which is considered a defect around the perimeter in the context of windshield damage. The first two inches around the perimeter has residual stress of about 1,000 psi from the bending and annealing process, with an original equipment windshield and over 1,000 psi on auto replacement glass (ARG) or after market windshields. This area fractures 2.5 times easier than the rest of the windshield. The second defect goes approximately 4 inches from the edge and is called induced or installation stress incurred from gluing the windshield to the vehicle. This causes a fracture in the residual stress area to crack immediately to the edge to relieve the induced stress and causes a gap at the edge of 0.0001 to 0.005. This gap is then closed by the lamination stress pulling the crack back together at around 10-12 inches from the edge. FIG. 2 schematically shows the gap of an edge crack in which 2a is the residual stress area and 2b is the induced stress area and 2c is the impact point now on each side of the crack. The gap and stress is why an edge crack must be repaired with multiple viscosity acrylic resins. This is also why 95% of edge cracks are long cracks or over six inches. The edge crack, which is almost always a long crack could not be successfully repaired by the windshield repair industry until inventions in U.S. Pat. No. 5,116,441 and CIPs. This was because the installation stress ranges from 1,000 to 3,000 psi and the windshield then standard low viscosity UV curing free radical acrylic resin did not have sufficient cohesive strength and shrunk upon curing which caused residual stress within the cured resin. This crack at one end is through to the edge and at the other end (in the middle section) is called the point and is repaired by drilling in front of the point, tapping a small bullseye and placing a repair tool at the point with a resin of low viscosity (20-90 cps). Crack openers will be placed on the inside to pry open the crack near the point and another tool will be mounted butted up to this tool with a medium viscosity resin of 200-400 cps. Another tool or just an injector held in the hand injects the edge crack resin, usually 1600 cps starting at the edge and going in toward the middle injections as far as it will go until the lamination stress pulls the crack tighter and tighter as is goes toward the point. The second tool with the medium resin is then slid to where the edge crack resin (1600 cps) stopped. A bead of resin is then placed on the surface of the crack and then Mylar tabs or strips are placed on top of resin to remove access to oxygen. The tool at the point is kept in place and then the first curing cycle is done. The tool at the point is left in the pressure mode to compensate for the shrinkage of the acrylic resin and the Mylar is to address the oxygen inhibition of the resin. After the resin has been cured the tool at the point is removed and Mylar is placed over the drilled hole and cured again. After the second curing the Mylar strips are removed, resin is scraped off the windshield and the mess is cleaned up. The Mylar tabs or strips will be a sticky mess and will be wiped off or placed in a jar of alcohol so they can be cleaned and used again or are thrown out. This is the most successful method to repair the edge crack today with three UV curing free radical acrylic resins, a 45-90 cps at the point, a 200-400 cps in the middle and a 1600-2400 cps at the edge area. The edge area with the induced stress (first four inches) is primed with a 45 cps resin when the glass is cold for wetting purposes.
Any air bubbles or voids in the repair prior to curing will upon curing go to the bottom along the laminate as the resin is crosslinking to itself and the glass. This spot which can start out as a centimeter becomes what is called a “runner” because it does just that, it runs for inches along PVB. It is a gap between the laminate and the cured resin. This will be seen after the repair is cured and the technician then does the process of removing a runner by drilling along side the crack in the middle of the runner which can be one to five inches long if too low of a viscosity was used.
The tabbing with Mylar can cause problems with air bubbles, especially when they are continually being reused and are dirty or bent. Because of the oxygen inhibition the air bubbles will cause a void and short chain links in the cross linkage of the radical cured acrylic resins which compromises the repair and results in a “sticky” surface of the resin. Using a “meet in the middle” method to try to eliminate one of the curing cycles (at the point) by using a 200 cps and 1600 cps resin where the 200 cps resin is placed at the point over the drilled hole and a mini bullseye tapped and then the 1600 cps at the edge is slid toward the middle until it stops somewhere in the middle. Then the 200 cps is slid to where the 1600 cps stopped, tools removed and then the crack is tabbed with Mylar and cured once. But this method leaves a void in the bullseye at the point because the tool with the 200 cps was not left on the point to cure under pressure and the 200 cps shrinkage causes a void that will refract and sometimes cause it to crack further from that imperfection. Sliding the 200 cps back to the drilled hole to cure under pressure would spot the crack. That is why the three resin method where one tool is placed at the point and left there through curing has been the most effective with the UV curing free radical acrylic resins. This method requires one or more crack openers on the innerlite and three tools on the outerlite, one at the point, one butted up to the tool with the medium resin and one at the edge with the edge resin. The tool at the point needs to be 45 cps so it can flow past the second tool butted up to it with the medium resin and then that tool is slide to the middle where the edge resin, usually 1600 cps stopped.
The interlayer Poly Vinyl Butyral i.e. PVB Film is a mixture of PVB resin and a plasticizer. PVB interlayers are manufactured at, and glass is fabricated with, an interlayer moisture level between 0.30% and 0.50%. PVB interlayers are also hydroscopic, which means they absorb moisture. According to PVB manufactures PVB, within 15 minutes of the glass fracturing and exposing the PVB to the air, will absorb moisture at the site of the exposure of the PVB from the atmosphere with relative humidity levels as low as 30%. In essence the exposed PVB comes with an inherent moisture content and sucks in moisture like a vacuum.
Although windshield repair is currently highly successful there is always room for improvement in making the process more efficient, less toxic, and easier to do and to strive for perfection. Here are the problems that the current free radical has and the cationic solves with the following windshield repair applications or processes: